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McCain said he would offer an amendment to the National Defense Authorization Act in September

McCain started his treatment for brain cancer on Monday

(CNN)Senate Armed Services Chairman John McCain on Monday threatened to present President Donald Trump with his own Afghanistan strategy if the Trump administration won't develop its own.

McCain issued a statement saying he would offer an amendment to the National Defense Authorization Act in September that would provide a strategy for Afghanistan, the 16-year war that has been a divisive issue within the White House.

"More than six months after President Trump's inauguration, there still is no strategy for success in Afghanistan," the Arizona Republican said. "Eight years of a 'don't lose' strategy has cost us lives and treasure in Afghanistan. Our troops deserve better. When the Senate takes up the National Defense Authorization Act in September, I will offer an amendment based on the advice of some our best military leaders that will provide a strategy for success in achieving America's national interests in Afghanistan."

McCain, who started his treatment for brain cancer on Monday, has long harped on Trump and top national security administration officials to provide Congress with a strategy for Afghanistan, where for years he slammed the Obama administration's timelines for withdrawing US troops from the conflict.

The Afghanistan plan has been delayed for months amidst sharp disagreements between national security adviser H.R. McMaster, who is arguing for an increase of several thousand troops to help turn the tide in the fight against the Taliban, and the President's chief strategist Steve Bannon, who is opposed to getting the US more deeply involved in the conflict.

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At a Senate hearing before McCain's committee in June, Defense Secretary James Mattis pledged to McCain that an Afghan strategy would be coming by mid-July, but the month is nearly over and a path forward has yet to materialize.

At the June confirmation hearing for Deputy Defense Secretary Patrick Shanahan, McCain fired a warning shot about his unhappiness with the administration on Afghanistan and his intention of providing the administration with a strategy if it couldn't come up with one.

"I want to work with this administration, I want to work with this President, work with new secretary of defense, who I happen to be one of the most ardent admirers of," McCain said. "But I have to tell you in a couple of weeks were going to mark up the defense authorization bill. The President has two choices: Give us a strategy, or we will put a strategy that we develop into the defense authorization bill."

McCain had hoped to take up the defense authorization bill, which sets Pentagon policy and authorizes spending, last week. But after his dramatic return and vote against the Republican Obamacare repeal plan, Sen. Rand Paul, R-Kentucky, objected to speedy consideration of the bill, and it had to be put off until September for McCain to take part in debate.

While McCain is starting cancer treatment that includes radiation and chemotherapy, he said in a statement Friday he plans to maintain a work schedule and then return to Washington after the August recess.